1. Field of the Invention
The present invention relates to an optical element molding method for molding an optical element such as a highly precise glass lens by pressure molding a softened glass material.
2. Related Background Art
With the reduction in size and weight of optical equipment, aspherical glass lenses are desired for use in the optical system. Since it is difficult to produce such lenses by ordinary polishing operation, there is being developed a method for producing an optical element without a post-working step such as polishing, by placing a glass material for producing the element such as a glass blank preliminarily molded to certain shape and surface precision between upper and lower mold members of a predetermined surface precision, and press molding the glass material under heating.
In such method for producing an optical element by press molding, as already disclosed in the Japanese Patent Publication No. 62-292636, a glass material is sandwiched between mold members for molding the optical element, then is heated, together with the mold members, to a temperature above the yield point of the glass and is pressed to the predetermined shape. Subsequently, the glass material is cooled to a temperature below the glass transition point while it is pressurized by the mold members, and is then taken out from the mold members to obtain a highly precise optical element. In this process, the mold members together with a cylindrical mold constitute a molding block, and the optical element is molded during transporting of the block to the stages of heating, pressing and cooling while the glass material is held in the block.
However, when the glass material is cooled to the temperature below the glass transition point while it is pressurized by the mold members, the mold members and the glass material are maintained in close contact, and, due to the difference in the contractions of the mold members and the glass material resulting from the difference in the thermal expansion coefficients thereof, a thermal stress is generated in the glass material. After the mold members are opened, the thermal stress is released, whereby a deformation is generated in the molded optical element.